There are two Global Navigation Satellite Systems (GNSS) which have been fully deployed for a number of years (the US Global Positioning System, the Russian GLONASS) and two more which are under deployment (the Chinese Beidou Navigation Satellite System and the European Galileo system). These systems rely on the same principles: microwave radio signals are broadcast from a number of satellites which orbit in a non-geostationary orbit; the signals carry a PRN (Pseudo Random Noise) code which is correlated with a local replica in a receiver configured to receive the broadcast signals; when a receiver is capable of acquiring and tracking signals from a satellite, its processing capabilities demodulate the code signal using the correlation process, and calculate a pseudo-range, which is the distance between the receiver and the satellite. This pseudo-range is taken in combination with pseudo-range acquired from other satellites (generally three) to determine a position, velocity and time (PVT).
Some of the radio navigation signals transmitted by the satellites are known as BOC signals (Binary Offset Carrier modulation), where a carrier wave is first modulated by a PRN code, and then by a subcarrier. The resulting signal has a spectrum having two main lobes located on either side of the carrier frequency, thus allowing cohabitation with other signals using the same carrier frequency. BOC signals are referred to as BOC(m, n), where the chip rate of the code signal is n*1.023 Mcps (Mega Chips per second), and the subcarrier frequency is m*1.023 MHz. These signals are selected for GNSS positioning instead of the traditional BPSK modulated signals because they show a better precision. Different variants of the BOC signal are used by Galileo and Beidou and will be used by the GPS 3 system.
The tracking of BOC signals has proven to offer more precise and robust positioning information than tracking of BPSK signals, mainly thanks to the sharper slope of the autocorrelation function peak, and its larger bandwidth. However, unlike BPSK signals, BOC signals autocorrelation function shows multiple side peaks which compete with the main peak, and show magnitudes comparable to the magnitude of the main peak.
In presence of error sources like noise or interferers, tracking of BOC signals may result in synchronizing on one of the side peaks of the BOC intercorrelation function. The tracking loop may therefore get locked at the right position, on the main peak of the intercorrelation, or at a wrong position, on a side peak of the intercorrelation, which creates a ranging error which can be higher than 9.7 m (case of BOC(15, 2.5)).
Positioning signals can also be affected by multipath, due to reflections on the environment occurring during the signal propagation. These multipath reflections are particularly present when operating in an urban or indoor environment. The reception of multipath signals creates artifacts in the composite intercorrelation function of a composite signal, multipath peaks being shifted from the original peaks by a distance corresponding to the delay between the main path and the multipath.
A number of state of the art techniques are dealing with the issue of synchronizing the tracking on the main peak of a BOC signal, but these techniques do not sufficiently consider propagation in a multipath environment.
Among these techniques, the Double-Discriminator technique, described in European Patent EP 2 382 484 B1, involves the parallel calculation of two discriminators, based on the spreading code and the subcarrier component of the BOC signal. The first discriminator calculation, called a non-ambiguous discriminator calculation, leads to a non-ambiguous determination of a tracking position, unlike the second discriminator calculation, which is ambiguous. However, the second discriminator calculation is more precise and less sensitive to noise and multipath reflections than the first one. In the Double Discriminator technique, a selection unit is configured to compare the value of the first discriminator with a threshold, and to select the discriminator value used in the tracking loop depending on the result of this comparison. If this technique is known to be very accurate in a Gaussian propagation environment, the performances deteriorate when the propagation environment show multipath reflections. The main reason for these deteriorations comes from the fact that multipath reflections particularly affect the shape of the non-ambiguous discriminator. Thus, the comparison of this discriminator value with a threshold shows a significant number of false alarms and non detections.